In a blender, combine the zest and juice of the lime and the orange with the 2 garlic cloves, oil, oregano, cumin, 1 teaspoon salt, and ½ teapsoon pepper.
Blend until smooth.
Score the flank steak with a few vertical cuts.
Place the flank steak either in a large plastic bag or glass baking dish along with all but ¼ cup of the marinade.
Marinate for 15 minutes or refrigerate and marinate up to 24 hours.
Light a charcoal or gas grill and make sure to oil the grate.
Remove the steak and shake off the excess marinade.
Season with salt and pepper on both sides.
Place on the grill.
Turn once and grill until lightly charred and an instant-read thermometer registers 135°, approximately 10-15 minutes.
Place the steak on a carving board and let sit for 5-10 minutes.
Slice the steak thinly against the grain and transfer to a platter.
Serve with the mango slices, lime wedge, and reserved marinade.
Notes
Make sure to set aside extra marinade for serving! Store leftovers in an airtight container for up to three days. If storing only the flank steak marinade, store it in an airtight container for up to a week.